In 1581 in Italy, Galileo saw a hanging lamp swinging and realised that it always takes the same amount of time to get from one side of the arc to the other no matter how large the arc is. He did lots of experiments and realised that a similar device, or pendulum, could be used to measure time. A pendulum is a piece of mass on the end of a string or wire that swings back and forth. All swinging pendulums eventually come to a stop, but lighter ones do it sooner than heavier ones.

Christiaan Huygens invented pendulum clocks in 1656. To measure time, a pendulum has to be a certain length. This meant that metal pendulums which expand in heat and shrink in the cold made clocks slower in summer and faster in winter. So pendulums were made of other things, such as mercury in a cylinder, which does not change size with temperature.